1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to disk drive servo control techniques that preserve position error signal (PES) continuity during track following operations.
2. Description of the Prior Art and Related Information
Today, computing devices such as personal computers, personal digital assistants, cell-phones, etc., are routinely used at work, at home, and everywhere in-between. Computing devices advantageously enable the use of application specific software, file sharing, the creation of electronic documents, and electronic communication and commerce through the Internet and other computer networks. Typically, each computing device has a storage peripheral such as a disk drive.
A huge market exists for disk drives for mass-market computing devices such as desktop computers and laptop computers, as well as small form factor (SFF) disk drives for use in mobile computing devices (e.g. personal digital assistants (PDAs), cell-phones, digital cameras, etc.). To be competitive, a disk drive should be relatively inexpensive and provide substantial capacity, rapid access to data, and reliable performance.
Disk drives typically employ a moveable head actuator to frequently access large amounts of data stored on a disk. One example of a disk drive is a hard disk drive. A conventional hard disk drive has a head disk assembly (“HDA”) including at least one magnetic disk (“disk”), a spindle motor for rapidly rotating the disk, and a head stack assembly (“HSA”) that includes a head gimbal assembly (HGA) with a moveable transducer head for reading and writing data. The HSA forms part of a servo control system that positions the moveable transducer head over a particular track on the disk to read or write information from and to that track, respectively.
Typically, a conventional hard disk drive includes a disk having a plurality of concentric tracks. Each surface of each disk conventionally contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo information may be provided on each disk to determine the position of the moveable transducer head.
The most popular form of servo is called “embedded servo” wherein the servo information is written in a plurality of servo sectors that are angularly spaced from one another and are interspersed between data sectors around each track of each disk.
Each servo sector typically includes a phase lock loop (PLL) field, a servo synch mark (SSM) field, a track identification (TKID), a sector ID field having a binary encoded sector ID number to identify the sector, and a group of servo bursts (e.g. an alternating pattern of magnetic transitions) which the servo control system of the disk drive samples to align the moveable transducer head with or relative to a particular track. Typically, the servo control system moves the transducer head toward a desired track during a “seek” mode using the TKID field as a control input.
Once the moveable transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the moveable transducer head over that track in a “track follow” mode. During track follow mode, the moveable transducer head repeatedly reads the sector ID field of each successive servo sector to obtain the binary encoded sector ID number that identifies each sector of the track. In this way, the servo control system continuously knows where the moveable transducer head is relative to the disk.
Position error signals (PESs) are an essential feedback signal for the servo control system during track following operations. The PES signal may be derived from the read servo bursts as the head flies over the servo bursts of the servo sectors of the disk. Various PES demodulation methods to calculate PES values from the bursts profiles are known.
Unfortunately, bursts ratio discontinuities, sometimes referred to as “glitches” may occur during PES demodulation. These PES glitches often introduce position errors into the servo control loop, degrading the performance of servo calibrations, and in extreme cases, may destabilize the track follow loop.